Navigation devices of the generic type are used, in particular, but by no means exclusively, in the form of mobile or permanently installed navigation devices in motor vehicles. The user has the option of inputting a destination, wherein the navigation device determines the current position of the motor vehicle by evaluating GPS-signals and subsequently directs the driver to the destination. Navigation devices of the generic type are equipped with a road network database that contains information on usable road segments. Navigation devices of the generic type, in particular, contain a road network database that also contains position information on tunnels. This tunnel position data is, in a general sense, referred to as tunnel attributes that are usually assigned to road segments.
When using tunnels, one encounters the problem that a very fast drop in the ambient luminous intensity occurs during the entry into the tunnel, particularly during daylight in the surroundings outside the tunnel. The much weaker ambient light in the tunnel may lead to a glare effect because the display unit of the navigation device which is usually realized in the form of an LCD monitor is adjusted to a luminous intensity or a color scheme that also makes it possible to recognize the road map and the position of the vehicle in daylight.
Various strategies for preventing this glare effect are known from the state of the art. DE 196 19 643 C1 describes a manual brightness control that enables the user to manually change the adjustment of the display unit to the requirements while driving through a tunnel by issuing corresponding user commands. This manual activation of the tunnel mode of the display unit has the disadvantage that it requires a substantial user input, particularly in very short tunnel sections, such that the unit is frequently not changed over and the excessively bright adjustment of the display results in a glare effect that impairs traffic safety while the vehicle drives through the tunnel. In addition, a manual change of the adjustment of the display unit requires much of the user's attention such that the user is distracted from the actual traffic events and, in turn, also impairs traffic safety. However, traffic safety is of particular importance in tunnels due to the higher risk of potentially severe traffic accidents.
As an alternative to the manual change-over of the display unit into a tunnel mode, there also exist initial solutions, in which the navigation device measures the ambient light intensity with a light sensor and varies the adjustments of the display unit in dependence on the measured ambient light intensity. For example, the brightness of the display can be increased in bright daylight and automatically decreased, for example, during nighttime hours or in a tunnel, based on the measuring signals of the light sensor. The utilization of a light sensor, however, has the disadvantage of additional hardware expenses. Furthermore, the integration of a light sensor into the hardware is not possible in some devices, for example, in mobile telephones or PDAs. In addition, the light sensor may result in faulty adjustments because the light sensor is constantly active and subject to interferences, for example, the dimmed headlights of oncoming vehicles, the tunnel lighting system or the interior lighting system of the vehicle. Consequently, it is hardly possible to adjust or program the light sensor such that it correctly adjusts the display brightness at the desired time only. A navigation device with light sensor is described, for example, in DE 32 45 299 A1 and DE 199 27 434 A1.
DE 196 19 643 C1 furthermore describes a strategy, according to which the tunnel mode is always activated when the current vehicle position is identified as a location that lies in a tunnel. This solution has the disadvantage that the tunnel attributes stored in the road network database are frequently not sufficiently accurate or definite. For example, data compression may cause tunnels and bridges to be provided with a common attribute (brunnel) such that the allocation of tunnels is no longer readily possible. In addition, semi-open tunnels or so-called galleries also carry the tunnel attribute, but do not require a change-over of the display unit to the tunnel mode because sufficient ambient light is incident through the gallery. Another major problem of systems in which the display unit is changed over into the tunnel mode based solely on the determined vehicle position and its identification as a tunnel can be seen in that the vehicle position in the tunnel needs to be extrapolated due to the lacking GPS signal if no other position finding sensors are available. The extrapolation preferably is carried out such that a vehicle position on a calculated route is assumed based on marginal conditions such as, for example, the speed before the GPS-signal is lost, the speed limit, the geometry of the map elements on which the vehicle travels, as well as the calculated route, wherein this vehicle position is then additionally processed in the navigation device in the form of an extrapolated GPS-position. This extrapolation of the vehicle position naturally has certain inaccuracies such that the actual vehicle position can significantly deviate from the extrapolated vehicle position. This deviation, in turn, results in the premature or delayed deactivation of the tunnel mode at the tunnel exit.